Surfacing nonimage areas of lithographic master with hydrophilic desensitizing composition

ABSTRACT

A convertible plate adapted for conversion into a lithographic offset master comprising a paper web, and a dielectric film extending over the paper web with finely divided silica distributed therein. In preparing an offset master, an imagedefining electrostatic charge is laid down on the film and developed with toner to form an image, and nonimage areas are rendered hydrophilic to prepare the plate for lithographic printing by coating the nonimage areas with a desensitizing composition comprising an aqueous solution of polyvinyl alcohol which mechanically fixed to the dielectric film through adhesion promoted by the divided silica.

United States Patent [72] Inventor Sangho E. Back Vancouver, Wash.

[2]] App]. No. 699,341

[22] Filed Jan. 22, 1968 [45] Patented Sept. 21,1971

I 7 3 Assignee Crown Zellerbach Corporation San Francisco, Calif.

[54] SURFACING NONIMAGE AREAS OF LITHOGRAPHIC MASTER WITH HYDROPHILIC DESENSITIZING COMPOSITION 7 Cla1ms,No Drawings [52] US. Cl 96/1, 96/l.5,96/1.8,96/33, 101/455, 101/460, 101/463, 117/135.1 [51] Int. Cl G03g 5/00, B4ln 1/00 [50] Field ofSearch 96/1, 1.5,

[56] References Cited UNITED STATES PATENTS 3,062,648 1 1/1962 Crawford 96/33 X 3,066,023 1 1/1962 Schlesinger 96/1.6 v

Hartsuch Chemistry of Lithography," 1961, LTF, Inc. pp. 123- 125 Primary Examiner-George F. Lesmes Assistant Examiner-R. E. Martin AttorneyKolisch and Hartwell ABSTRACT: A convertible plate adapted for conversion into a lithographic offset master comprising a paper web, and a dielectric film extending over the paper web with finely divided silica distributed therein. In preparing an offset master, an image-defining electrostatic charge is laid down on the film and developed with toner to form an image, and nonimage areas are rendered hydrophilic to prepare the plate for lithographic printing by coating the nonimage areas with a desensitizing composition comprising an aqueous solution of polyvinyl alcohol which mechanically fixed to the dielectric film through adhesion promoted by the divided silica.

SURFACING NONIMAGE AREAS OF LITHOGRAPHIC MASTER WITH HYDROPHILIC DESENSITIZING COMPOSITION This invention relates generally to the preparation of a lithographic printing plate or offset master by electrostatic procedures, and more particularly concerns the preparation of such a printing plate where proper hydrophilic properties in nonimage areas of the plate are obtained by a desensitizing composition distributed over such nonimage areas which is hydrophilic in nature. The desensitizing composition fixes itself to the plate by adhesion promoted by the inclusion of silica in the dielectric film which forms part of the plate.

In lithographic printing, a plate is prepared with image areas on the surface of such plate oleophilic or hydrophobic, and with nonimage areas hydrophilic. Such a plate may be wet with an aqueous fountain solution, which functions to wet the hydrophilic nonimage areas exclusively of the image areas, and when such a plate is then contacted with an ink roll coated with an oil-base printing ink, such ink deposits only on the oleophilic areas of the plate. The ink may then be transferred to a paper sheet to produce a final print.

Several methods have been proposed for preparing an image on a plate to be used in lithographic printing using electrostatic printing principles. In one type of procedure, referred to herein as electrophotographic imaging because of its resemblance to an electrophotographic printing process, a plate or sheet including a photoconductive layer extending over its face is first given a uniform charge over its entire surface in the dark. Then an image-defining electrostatic charge may be formed by exposure of the charged photoconductive layer to light, through a positive image, with the charge being removed in those areas receiving light and remaining where not received, much like a photographic process. In another type of electrostatic process, referred to herein as electrographic imaging because of its resemblance to electrographic printing, an image-defining electrostatic charge is laid directly down upon an insulating or dielectric film in a plate, using such electronic devices as a cathode-ray tube, a pin matrix and pulsing corona discharge, or by means of a direct image transfer from one surface to another. With either of such electrostatic imaging procedures, after the image-defining charge has been formed on the sheet, the image may be developed using a finely divided developer material or toner, which deposits on the sheet in those areas where the charge appears. The sheet or plate with the image developed thereon may be employed as the offset master in a lithographic printing process, providing the sheet, either after development or after further processing, has proper hydrophilic-oleophilic balance, with the image areas being oleophilic, and the nonimage areas being hydrophilic.

In a more particular sense, the invention concerns the preparation of a lithographic printing plate using an electrographic imaging procedure. The dielectric film which receives the charge ordinarily contains minor amounts at most of any photoconductive material such as zinc oxide, and preferably is relatively thin and highly dielectric in nature. With such a film good image resolution and density, with minimal dissipation of the charge on the film by lateral leakage, are obtained.

A problem arises in the preparation of a convertible plate to be converted by electrographic imaging into an offset master in that most, if not all, known resins which are highly dielectric in nature are also hydrophobic. Thus, any dielectric film prepared from such resins must be desensitized in nonimage areas if the film in nonimage areas is to be properly wettable with an aqueous fountain solution.

In general terms, this invention concerns a novel plate convertible into an offset master using electrostatic imaging procedures, wherein the dielectric film which surfaces the plate includes finely divided silica dispersed therein which provides a very fine roughing on the outer surface of the dielectric film. After the development of an image, a hydrophilic desensitizing composition is spread over nonimage areas, with such fixing through what is referred to herein as a mechanical bond with the finely roughened surface of the dielectric film. The desensitizing composition remains distributed over nonimage areas during the process of making lithographic prints, which involves repeated wetting of the nonimage areas with an aqueous fountain solution followed by the application of ink to image areas.

The invention also concerns a novel method for rendering nonimage areas in a plate to be used in lithographic printing hydrophilic, whereby such will suitably be wetted with a fountain solution during a lithographic printing procedure.

The silica which is distributed in the dielectric film is finely divided and also has a relatively large surface area in relation to its size. Thus, an amorphous silica aerogel is contemplated, which comprises very irregularly shaped particles having a large surface area. The large surface area promotes the mechanical bonding of the desensitizing composition to the dielectric film. I

A still further object of the invention is to provide a method for desensitizing a convertible plate which utilizes the application of a thin spread of aqueous polyvinyl alcohol solution over the dielectric film after the development of an image, such polyvinyl alcohol material being hydrophilic and through a mechanical bonding action becoming semipermanently anchored or fixed to nonimage areas thus to provide proper hydrophilic properties in the plate during the duplicating process. Silica exhibits an affinity for the polyvinyl alcohol, by reason of the multiple, hydroxyl radicals in the alcohol, which contributes to the mechanical bonding of the spread to the dielectric film. I

These and other objects and advantages are attained by the invention, and the same will be described hereinbelow in conjunction with certain specific examples, to be considered as illustrative of the invention.

As contemplated herein, the plate may be prepared from a paper sheet or web which provides support for various coatings subsequently applied thereover. There are no particular requirements for such web, other than that it have sufficient strength to withstand the operations to which the plate is subsequently subjected to make an offset master frornit, and such requirements as are dictated by economic reasons, etc. Plates such as metal plates could alternativelybe used, although such are more expensive and are not as well suited for use in a disposable-type product.

The paper sheet or web is prepared into what is referred to herein as a sheet base by applying over one face of the web a conductive coating of relatively electrically conductive material. Preferably there is applied over such conductive coating a so-called holdout coating, which by covering the conductive layer and web inhibits impregnation of the web by the resin constituents forming the dielectric film which is spread over the holdout coating. The sheet base is normally supercalendered, to prepare it for the reception of the coating material which forms the dielectric film in the convertible plate.

Optimum conductivity in the sheet base is realized with a resistance (surface resistivity) in the sheet base lying somewhere within the range of 10 to 10 ohms per square. .Witha resistance below this range (or a greater conductivity), image retention problems are introduced, due to loss, of charge, and with a resistance above the range indicated, stray electrical charges are not sufficiently well dissipated to produce optimum image resolution.

Illustrative of conductive coating formulations thatmay be employed are aqueous solutions of the so-called conductive resins, exemplified by vinylbenzyl quaternary ammonium chloride resin, and so-called salt-bridge type of solutions, exemplified by an aqueous solution of equal parts lithium chloride in glycerin. Enough water is included to obtain the fiowability desired for spreading on the paper. Spreads of such relatively conductive materials are usually quite thin, with spreads of between 0.5 and 2.0 pounds per ream being typical (on a solids basis). As used herein, a ream refers to 3,000 square feet of paper.

As briefly discussed above, the holdout coating which is applied over the conductive layer functions to prevent penetration of the sheet base with the spread used in making the dielectric film. Spreads of from 1 to 5 pounds per ream, based on a solids basis, are usually used. Various formulations have been employed in the preparation of such a holdout coating, including mixtures of a mineral pigment such as clay and polyvinyl alcohol dispersed in water, mixtures of starch and mineral pigment dispersed in water, etc.

The dielectric film which is prepared over the sheet base should have a resistance ranging upwardly from about ohms per square, in order to obtain proper image resolution. The film, as has earlier been discussed, includes a finely divided silica distributed therein, which performs at least several functions in the dielectric film. Further explaining, silica is quite inert, and when distributed in a dielectric film does little to impair its dielectric properties. Further, silica exhibits what might be thought of as hydrophilic characteristics, since silica tends to be easily wetted with water. Of primary importance, however, is the fact that by including finely divided silica in the dielectric film, a film results which exhibits considerable affinity for a hydrophilic desensitizing composition such as a polyvinyl alcohol solution. A polyvinyl alcohol solution, on being distributed over a film including dispersed silica, tends to remain semipermanently fixed to the film, whereby it does not wash away after repeated application of fountain solution. The amount of silica which a dielectric film may have included in it is limited by the fact that with a proportion of silica to resin (the binder) in the film exceeding about 1 to 2 (i.e., 50 percent by weight of the resin binder in the film), the film becomes crumbly and will not hold together properly on the sheet base. To realize significant advantages from the silica, it should be present in an amount exceeding about 10 percent by weight of the resin binder in the film. Thus, it is the affinity of a dielectric film including a limited amount of silica, i.e., an amount not sufficient to render the film crumbly, for the desensitizing solution, which is the important part of this invention, as by reason of this affinity the hydrophilic characteristic of the film can be masked easily and in a manner which is permanent enough to withstand repeated treatments with fountain solution.

Silica aerogel is a finely divided amorphous substance, having a particle size usually below about 0.2 microns, and because of the irregular thin walled nature of the particles, the material has a relatively large surface area for a given quantity of material. Thus, a surface area of more than 50 square meters per gram of material can be expected, with some silica aerogels having surface areas up to about 275 square meters per gram. These characteristics of the aerogel make the material particularly useful in practicing this invention.

Exemplary of coating compositions that may be utilized in the preparation of the dielectric film are organic solvent solutions (i.e., methylethyl ketone, toluene, Fotocol (commercial ethyl alcohol), etc.,) of polyvinyl acetate, diluted to about 25 percent by weight solids content to permit spreading, and including from to 30 parts silica for 100 parts polyvinyl acetate. Alternatively, styrenated alkyd resins may be utilized, as well as such resins as butadiene, styrene acrylate, sarans, and acrylic and methacrylate resins, polyesters, etc. Preferably, and when alkyd resins, for instance, are utilized, because such tend to be soft, an acrylic resin is included in the coating composition to toughen the film formed. Ordinarily, the coating compositions are diluted with enough solvent to permit easy spreading, i.e., to a solids content of from to 40 percent by weight. Spreads of from about 5 to 16 pounds per reamare usual (based on solids content). in addition to silica,

the dielectric film may also include mineral pigment such as zinc oxide, titanium oxide, zinc sulfide,.etc. Titanium oxide, for instance, is useful in imparting whiteness to the plate. Ordinarily such other mineral pigments are included in concentrations not exceeding about 50 percent by weight of the resin in the film, as such tend to impair the dielectric properties of the film.

According to this invention, and after the convertible plate has had the image-defining electrostatic charge applied to it, and the charge areas resulting have been developed by the application of toner with fixing of the toner, the plate is con verted into one suitable for lithographic printing by swabbing nonimage areas of the dielectric film with the polyvinyl alcohol desensitizing solution to form a coating on the film. The desensitizing solution is chiefly water, with from about 0.2 to about 1.5 percent by weight polyvinyl alcohol dissolved therein. A greater amount of polyvinyl alcohol tends to render developed image areas hydrophilic with unclear prints resulting. Where a mineral pigment such as zinc oxide is included in the dielectric film, the desensitizing solution may also include from about 0.1 to about 5.0 percent by weight of a water-soluble ferrocyanide, and an acid in a concentration to give the solution a pH equal to that caused by the addition to the solution of from about 0.1 percent to 2.0 percent (by volume) of glacial acetic acid. Such an acetic ferrocyanide solution forms a soluble complex with the mineral pigment, whereby the dielectric film is etched with application of the desensitizing solution. Etching enhances mechanical fixing of the polyvinyl alcohol. It is also sometimes desirable to include in the desensitizing solution up to about 15 percent by weight glycerin, such being a plasticizer and operating to inhibit curling of the convertible plate (sometimes a problem when the sheet base is formed of paper).

Explaining a specific example of the invention, conventional 30-lb. label base paper was utilized as the paper web in preparing the sheet base. A conductive coatingwas applied over such web of paper from a coating composition comprising parts Dow QX 2611.7 (an aqueous solution of vinylbenzyl quaternary ammonium chloride, from Dow Chemical Corporation, 40 percent solids), 15 parts Borax, and sufficient additional water to bring the solids content of the solution to 27 percent. The borax was included to fonn a gel on the inner face of the holdout coating subsequently applied over the conductive coating, with such gel inhibiting impregnation of the paper web by the holdout coating. Direct steam was used to dissolve the borax. The coating composition was spread on the paper using a spread of about 0.7 pounds per ream (on a solids basis). The coating prepared had a resistance of about l0 ohms per square, at 50 percent humidity and 70 F.

A holdout coating was applied over the conductive coating, from a coating composition comprising a mixture of 100 parts mineral pigment (clay) and 50 parts polyvinyl alcohol, dispersed in 600 parts of water. Such was applied with an airknife coater, using a spread of about 2 to 3 pounds per ream, on a solids basis. On drying of the holdout coating, the sheet base was supercalendered.

With the sheet base prepared, a dielectric film was prepared over the sheet base from a coating composition comprising 75 parts alkyd resin (styrenated copolymer of phthalic anhydride and glycerin), 25 parts polymethylmethacrylate resin, 20 parts silica aerogel (Monsanto Santocel FRC), 25 parts zinc oxide, and sufficient organic solvent (toluene) to produce a 35 per cent by weight solids composition. The silica aerogel had a surface area per gram of about 250 square meters, and a particle size ranging from 0.01 to 0.02 microns. Such coating composition was spread at the rate of about 10 pounds per ream, on a solids basis. A dielectric film was produced having a resistance of about 10" ohms per square.

An image-defining electrostatic charge was laid down on the convertible plate produced using electrographic imaging procedures, more particularly, by direct image transfer of the image from one surface to another. Conventional oleophilic developer material, i.e., toner, was then distributed over the dielectric film or sheet, with such adhering to the charged areas of the sheet, and such toner was cured by heating the plate in an oven. For a description of toners suitable for development of the image, reference is had to US Pat. No. 3,107,169, to Bornarth entitled Processes of Producing Lithographic Electrostatic Printing Plates.

With the toner cured, nonimage areas of the plate were desensitized or rendered hydrophilic, whereby the plate was converted into an offset master suitable for lithographic printing, by swabbing the nonimage areas with desensitizing solution comprising 2 parts sodium ferrocyanide, 2 parts concentrated hydrochloric acid, 10 parts glycerin, 10 parts aqueous 10 percent solids polyvinyl alcohol solution, and 76 parts water. The polyvinyl alcohol was a low viscosity material (viscosity below 6 Centipoises in 4 percent solids aqueous solution at 70 F.). Etching of the nonimage areas in the dielectric film was noted, by reason of the zinc oxide in the dielectric film being converted to a soluble zinc complex with the desensitizing solution. A film of polyvinyl alcohol remained distributed over and mechanically fixed to the nonimage areas of the dielectric film.

A master so prepared was used in the production of lithographic prints, using conventional lithographic printing procedures, wherein the nonimage areas of the plate are first wetted with an aqueous fountain solution. After application of fountain solution, an ink roll coated with an oil-base printing ink deposited ink on the image areas, and the ink on such image areas was then transferred to a rubber blanket which, in turn, transferred the ink to a sheet of paper to form a print. 50 prints were made in such manner, with good prints resulting. The desensitizing solution remained distributed on the plate throughout the printing process.

Another convertible plate was prepared with the same conductive layer as in the previous example and with a holdout layer in this instance prepared from a coating composition comprising 100 parts clay, 50 parts starch, and 300 parts water, applied at the rate of about 4 pounds per ream, on a solids basis. Over the holdout coating a dielectric film was prepared from a coating composition comprising 100 parts polyvinyl acetate, and 20 parts silica aerogel in 360 parts Fotocol solvent (a commercial grade of ethyl alcohol containing about 20 percent water). The composition was spread at the rate of about 10 pounds per ream on a solids basis. After imaging and development of the image, a desensitizing solution, comprising an aqueous polyvinyl alcohol solution (1 percent solids) was applied over the nonimage areas of the dielectric film. The offset master resulting was used in the production of 50 good prints.

From the above description it should be obvious that a novel type of convertible plate and a novel form of offset master are contemplated, as well as a novel method of desensitizing a plate to render it suitable for lithographic printing. The method features the distribution of a hydrophilic constituent, i.e., polyvinyl alcohol, over the nonimage areas of the dielectric film with the dielectric film mechanically retaining the polyvinyl alcohol after its application.

While specific examples of this invention have been disclosed, it should be obvious that modifications and variations are possible without departing from the invention. It is desired to cover all such modifications that would be apparent to one skilled in the art, and that come within the scope of the appended claims.

It is claimed and desired to secure by letters patent:

1. An offset master for lithographic printing comprising a plate including a sheet base and a resinous dielectric film extending over said sheet base, said sheet base in comparison with said film being relatively conductive, said dielectric film having a resistance greater than about 10 ohms per square and comprising 100 parts resin binder and from to 50 parts finely divided silica distributed in the resin binder, and a film of polyvinyl alcohol extending over areas of said dielectric film and adhering to the latter film, said polyvinyl alcohol film rendering such areas hydrophilic whereby such areas are wettable with an aqueous fountain solution.

2. The offset master of claim 1, where the dielectric film further comprises up to 50 parts zinc oxide.

3. The offset master of claim 1, wherein the silica is silica aerogel, with a surface area exceeding 50 square meters per g ram.

4. A method of preparing a lithographic offset master from a convertible plate comprising a sheet base and a dielectric film extending over the sheet base where the sheet base is relatively conductive in comparison with said film, said dielectric film having a resistance above about 10 ohms per square and being adapted to receive an image-defining electrostatic image charge, said dielectric film further including finely divided silica distributed in the film, comprising 100 parts resin binder and from 10 to 50 parts finely divided silica, the method comprising depositing an image-defining electrostatic charge on the dielectric film to produce image charge areas thereon distributed with nonimage areas, developing the image charge areas with a developer material which covers and fixes to the dielectric film in said image charge areas while leaving nonimage areas free of developer material, and coating the dielectric film in such nonimage areas with a film of polyvinyl alcohol, such film of polyvinyl alcohol serving to render such nonimage areas hydrophilic and capable of being wetted with an aqueous fountain solution.

5. The method of claim 4, wherein the film of polyvinyl alcohol is prepared by distributing over the dielectric film in nonimage areas a desensitizing solution comprising water and up to 1.5 percent by weight polyvinyl alcohol.

6. The method of claim 4, wherein the film of polyvinyl alcohol is prepared by distributing over the nonimage areas in the dielectric film a desensitizing composition comprising water, up to about 1.5 percent by weight polyvinyl alcohol, and up to about 15 percent by weight glycerin.

7. An offset master for lithographic printing comprising a plate including a sheet base and a dielectric film extending over said sheet base, said sheet base in comparison with said film being relatively conductive, said dielectric film having a resistance greater than about 10 ohms per square and comprising 100 parts resin binder and from 10 to 50 parts finely divided silica distributed in the resin binder,

a fixed toner deposit on the surface of the dielectric film extending over image areas in the master, and

a film of polyvinyl alcohol extending over nonimage areas of said dielectric film, said polyvinyl alcohol adhering to the silica distributed in the resin binder of the dielectric film and rendering the nonimage areas wettable with an aqueous fountain solution. 

2. The offset master of claim 1, where the dielectric film further comprises up to 50 parts zinc oxide.
 3. The offset master of claim 1, wherein the silica is silica aerogel, with a surface area exceeding 50 square meters per gram.
 4. A method of preparing a lithographic offset master from a convertible plate comprising a sheet base and a dielectric film extending over the sheet base where the sheet base is relatively conductive in comparison with said film, said dielectric film having a resistance above about 1010 ohms per square and being adapted to receive an image-defining electrostatic image charge, said dielectric film further including finely divided silica distributed in the film, comprising 100 parts resin binder and from 10 to 50 parts finely divided silica, the method comprising depositing an image-defining electrostatic charge on the dielectric film to produce image charge areas thereon distributed with nonimage areas, developing the image charge areas with a developer material whiCh covers and fixes to the dielectric film in said image charge areas while leaving nonimage areas free of developer material, and coating the dielectric film in such nonimage areas with a film of polyvinyl alcohol, such film of polyvinyl alcohol serving to render such nonimage areas hydrophilic and capable of being wetted with an aqueous fountain solution.
 5. The method of claim 4, wherein the film of polyvinyl alcohol is prepared by distributing over the dielectric film in nonimage areas a desensitizing solution comprising water and up to 1.5 percent by weight polyvinyl alcohol.
 6. The method of claim 4, wherein the film of polyvinyl alcohol is prepared by distributing over the nonimage areas in the dielectric film a desensitizing composition comprising water, up to about 1.5 percent by weight polyvinyl alcohol, and up to about 15 percent by weight glycerin.
 7. An offset master for lithographic printing comprising a plate including a sheet base and a dielectric film extending over said sheet base, said sheet base in comparison with said film being relatively conductive, said dielectric film having a resistance greater than about 1010 ohms per square and comprising 100 parts resin binder and from 10 to 50 parts finely divided silica distributed in the resin binder, a fixed toner deposit on the surface of the dielectric film extending over image areas in the master, and a film of polyvinyl alcohol extending over nonimage areas of said dielectric film, said polyvinyl alcohol adhering to the silica distributed in the resin binder of the dielectric film and rendering the nonimage areas wettable with an aqueous fountain solution. 